The present invention relates to a method for a motion compensating field interpolation as well as the use of this method. Such a method is particularly suitable for the reconstruction of omitted fields of television sequences.
The problem of interpolating fields in digital television sequences arises in the case of field frequency conversion. In source coding applications, fields which are dropped in the coder to allow for a transmission of television signals in digital channels with very low transmission bit rates have to be reconstructed. The CCITT Study Group XV is currently investigating television codecs with 384 kbit/s transmission rate. The aim is to provide video conference services using the Integrated Services Digital Network ISDN. In order to achieve this data rate, it seems to be necessary to reduce the number of transmitted fields in addition to known source coding techniques. A recently presented paper on this subject (G. Kummerfeldt et al, "Coding Television Signals at 320 and 64 kbit/s", 2nd Int. Tech. Symposium on Optical and Electro-Optical Applied Science and Engineering, Cannes, December 1985) uses a field subsampling by a factor of 4:1 combined with a hybrid coding algorithm. Then, at the receiver, three fields have to be interpolated between every two transmitted fields.
The problem is to generate a field at a given temporal position between two successive fields of a television sequence. Since the temporal luminance changes are often times caused by moving objects, a simple field repetition technique, which inserts the nearest available field, yields jerkily moving objects (see B. G. Haskell et al, "A low bit-rate interframe coder for videotelephone", Bell Syst. Techn. J., Vol. 54, No. 8, pp. 1475-1495, October 1975). Another simple approach is linear interpolating by temporal filtering (see J. Klie, "Codierung von Fernsehsignalen funiedrige Ubertragungsbitraten", Ph.D. dissertation, Tech. Univ. of Hannover, Germany 1978). In this case for each picture element to be interpolated, a weighted average of the corresponding picture elements at the same spatial position is calculated. This interpolation technique is able to adapt to luminance changes which, for example, are only due to illumination changes of the scene contents. However, blurring becomes visible in moving areas depending on the amount of motion.
Hence, motion compensating interpolation (MCI) techniques have been developed, which were reviewed in a paper by H. G. Musmann et al, "Advances in Picture Coding", Proc. of the IEEE, Vol. 73, pp. 523-548, April 1985. MCI-techniques take into account the motion of objects to preserve the natural impression of motion. The main difficulty is to provide a sufficiently precise estimation of the motion parameters. In order to limit the complexity of the algorithm, most of the MCI-techniques are based on the assumption of pure translatorily displaced objects in the image plane, for example, see the Kummerfeldt et al paper, the paper by J. R. Jain et al, "Displacement measurement and its application in interframe image coding", IEEE Trans. on Comm., Vol. Com-29, No. 12, pp. 1799-1808, December 1981, the paper by A. Furukawa et al, "Motion-adaptive interpolation for videoconference pictures", Proc. of the Int. Conf. on Comm., 1984, pp. 707-710, Amsterdam, 1984, and the paper by H. C. Bergmann, "Motion adaptive frame interpolation", Proc. of the 1984 Int. Zurich Seminar on Digital Communications, D2.1-D2.5, Zurich, 1984, as well as the presented technique. In the Kummerfeldt et al and Jain et al papers, the images are subdivided into a fixed number of rectangular blocks. One displacement vector is determined for each block belonging to a moving area, whereas in the Furukawa et al paper only one representative displacement vector is determined for each moving area. The algorithm described in the Bergmann paper proposes the estimation of one displacement vector for each picture element in the moving image parts to obtain an improved rendition of motion in the reconstructed television sequence.